Much have I travell'd in the realms of gold, And many goodly states and kingdoms seen; Round many western islands have I been Which bards in fealty to Apollo hold. Oft of one wide expanse had I been told That deep-brow'd Homer ruled as his demesne: Yet did I never breathe its pure serene Till I heard Chapman speak out loud and bold: Then felt I like some watcher of the skies When a new planet swims into his ken; Or like stout Cortez, when with eagle eyes He stared at the Pacific - and all his men Look'd at each other with a wild surmise - Silent, upon a peak in Darien.

We "did" this poem at school - and even to my schoolboy's eyes it seemed an unlikely brew. A hotchpotch of names - Keats, Chapman, Homer: they bounce the reader through literature, through time. A giddy geography - we're swept from Apollo's Aegean to Cortez on his Pacific shore. We begin with a traveller who tells of ancient bards, he briefly tries his hand at astronomy, till "silent, upon a peak" he's transformed to the "stout" conquistador. Three poems have we here: Homer's original Iliad, Chapman's rendering of this into English, and rounding it all off, on the page before us, Keats's wonderment on "first looking into" Chapman's translation. All this in 14 lines.

The schoolboy is quite stupefied and who can blame him if once more his arms fold on his desk, he daydreams out the window. Outside it is raining, as it always is for English class on Friday afternoons. The pigeons troop to the windowsill, drooping under the shelter. And slowly a smile forms on the schoolboy's face. On first looking into Chapman's Homer... of course! He giggles; nudges his deskmate to share the joke. Chapman's homer, do you see? - it's a pigeon. A chalk grazes his ear, fired from the blackboard where the teacher glowers.

But the glory of Brian O'Nolan is that he did pay attention, a precise delightful attention to words, and the fantastical whimsy -given leash - they will convey.

Brian O'Nolan was his given name. Sometimes he rendered it in Irish: Brían Ó Nualáinn. Nowadays, he is best known to us as Flann O'Brien, the soubriquet under which he published his novels, which range from the 1939 classic, At Swim-Two-Birds, to The Third Policeman, published posthumously in 1967. In his lifetime, however, he was celebrated by yet another pseudonym - Myles na Gopaleen - under which he wrote his "Cruiskeen Lawn" column for the Irish Times, whence The Various Lives of Keats and Chapman is taken.

At Swim-Two-Birds had issued upon a world somewhat preoccupied with the outbreak of the second world war. It sold perhaps 300 copies, but with the support of luminaries such as Graham Greene in London and James Joyce in Paris, it began to be thought a "critical success" - that happy formula for any aspiring writer.

Critical success, to be convincing, must adumbrate an actual success. O'Brien believed he had provided for this in 1940 when he completed the manuscript of The Third Policeman. Ironically, for it is now his most widely read novel, it was rejected by the London publishers. For the rest of his life, Flann or Brian or Myles (whichever he currently was) excused the non-appearance of this masterpiece by the disappearance of its manuscript, variously explaining to his drinking cronies that he had lost it in a pub or mislaid it on a train.

The fobbery of lost manuscripts is the stuff of Dublin legend: but this particular manuscript was found, after the author's death, on a sideboard in his home where it had lain apparently undisturbed and in daily view for 26 years, a mocking reminder of the London houses' rejection - of a masterpiece, and of him, too, as its author, we may presume.

He turned to journalism (or more correctly, columnism), and over the next 20 years, as Myles na Gopaleen, he was the scourge of pretension throughout Ireland, most particularly in the rising governmental class - and at a time, let us be plain, when it was neither popular nor profitable so to be.

The milieu is the Dublin of the 1940s and 1950s, a city hard now to conjure save in the tones of grey and grey-green. We picture the lumbering Liffey and the clouds trundling overhead. We visit the quayside haunts - the Palace Bar, the Scotch House - mix with the clientele, behatted, besuited, shabbily genteel. The pints of plain are flowing, along with the balls of malt. We listen to the jokey banter that passes for intellectualism. It's a smoky masculine world: women scarcely exist save to mother or to serve. Around us Dublin is rotting, dear old dirty Dublin. Beyond lie the wastes of sadness, whence all brightness is exported in the emigrant ships to Liverpool, New York. The paralysis of Joyce's Dubliners had briefly been jolted by the fireworks of independence: now conformity reigns. It turns out that Ireland's 800-year struggle had not been for freedom at all: merely for separation, a wall against the new.

"The Cruiskeen Lawn" was originally conceived as an Irish-language column for the Irish Times - an unlikely home for such an enterprise, as that paper was still largely Unionist in outlook, though by the 1940s it condescended to nod to the new civil order. The column soon outgrew its roots and within a year Myles was writing predominantly in English, though he still made the odd sortie into Irish (and into punning Latin and modern European languages too, if the maggot bit). At odd times he wrote in a mocking miachstúir of English-Irish or English speilt as Irish.

The column grew, over the years, to a sustained chronicle of wit and whimsy, a treacle well of satire. The running jokes are legendary: the Brother, that quintessential Dubliner (or is it the brother of the Brother who is truly quintessential?); the Plain People of Ireland with their proprietorial demands to be told; the Research Bureau and its absurd inventions; the Catechism of Cliché, whose form, if not its fun, may be traced to Aquinas. A personal favourite is the Book-Handling Service, whereby for a small fee, Myles's team will attend the client's home, fox the pages of his books, scribble on them even, underline obscure but pertinent passages, thus affording the busy bourgeois the appearance of a well-read man.

It became the required reading of the Dublin intellectuals, that inward coterie of wags and pranksters (among whom Myles undoubtedly numbered himself) who, like the Free State, were victims of promise and for whom, in the clerical orthodoxy that was Eire, promise was safer than success. It reflected their preoccupations, but simultaneously commented upon them: for all affectation, even his own, was grist to Myles's mill. Pomposity, the sham of authority, jobbery of the state: these were his quarries. A balloon might not swell but Myles's pen was there to pierce it. His attacks might be short or sustained, but their surest end was if someone Myles disapproved of joined in the assault: whereupon he would lampoon his unlucky and interim ally with all the scorn previously reserved for the original target.

If there is any philosophy here at all, it is that we are all much of a muchness and nowhere is much better than anywhere else. The great battles have been fought, our humanity was long ago weighed in a sort of Thomist summation.

Nothing much needs to be added, save perhaps St Thomas might have underestimated the extent of evil in this world and the fallen state of its inhabitants. Progress is not only illusory but impractical. Knowledge, particularly scientific knowledge, is no more than intellectual recreation.

It has been called Manichaeism, and there is in much of O'Brien's and Myles's writing a sense of the arcane, a cabalistic insistence on number, knowledge as gnosis, as ritual. The truth is Brian O'Nolan remained a Catholic, believing and practising, all his life. And the Catholic church - surely the most bloated balloon of all - was rarely the butt of his jokes.

None of this is to detract from the comedy of the column, the sheer hilarity of its invention. But humour is a serious business: nonsense has its purpose. Its purpose, for Brian O'Nolan, was to disagree - the prerogative, and perhaps the province, of all writers.

He died in - but let us rather, in the style of his column, play a short game of Catechism.

What bread did Brian O'Nolan earn?

His daily bread.

And how was such necessary sustenance procured?

By following his father into the Irish public service.

To what exalted position did he eventually rise?

Principal officer for town planning.

And under what did he resign?

Under pressure.

State the rigorosity of said pressure.

Severe.

Was it severe pressure brought on by ill-health and the thirsty pursuit of too many balls of malt?

That, and the rancour of the powers-that-were who would no longer abide a civil servant lambasting them in the daily press.

Did he die in a state of happy fulfilment?

No, he died in 1966.

Give the standard life.

Anthony Cronin, No Laughing Matter: the life and times of Flann O'Brien ; Grafton Books, London, 1989.

But the best game of all, played now on websites, is the atrocious, execrable punning of The Various Lives of Keats and Chapman.

In his dictionary Johnson defines "conundrum" as a low jest; a quibble; a mean conceit. But, in passing, Johnson was no mean hand at the pun himself. Here is my favourite. Two termagants are quarrelling across a lane from tenement windows. "They'll never agree," Johnson remarks, "for they're arguing from different premises."

And there you have the essence of the pun: the double meaning and the extravagant clash of ideas. Too well is it called a clench, for we do feel an involuntary musculation, a clinch of recognition as though our buttocks truly had squeezed.

It is perhaps only in Ireland that the pun is given its proper due. Listen to the banter in any Dublin pub: pun upon pun spiralling in wafts of surreality to hang under the ceiling in the cigarette smoke, "that incense", as a friend described it, "which heaven doesn't want". In Ireland we have a playfulness with our English. The language is not a national treasure. It's a borrowed thing, a loaned vocabulary on an older Gaelic syntax. Picture it and you see peasants cavorting in the Big House, chopping up the Chippendale for low use on the fire. We have a child's delight in vocabulary, as though the novelty has yet to wear off.

There is a certain anarchy, too, in the Irish make-up, a lust for licence, an anti-authoritarianism. And a corresponding love of rules - the better to know how to break them, perhaps. This dazzles in the fireworks of Joyce, even in Wilde. But it is in Flann O'Brien (or Myles na Gopaleen) that it finds its comic exemplar.

So how do we approach these tall tales? Well, on first looking into Myles's Keats and Chapman, we're struck by their fantastic, almost Gothic, structure. We find our two heroes in the most unlikely circumstances. They are strolling players in France, explorers on behalf of the Royal Society, they go beer-tippling in the south of England, they supervise the construction of the Zurich tram-car system. They are duellists, biochemists, carnival showmen, amateur physicians, potato factors, economists of the Manchester School. They attended Greyfriars together. They visit the Vale of Avoca. Was ever in English letters a comparable duo?

Well, of course not. The direction of the extravagance is the pay-off of the pun. But its purpose is the extravagance itself. It goes without saying that these tales were written backwards. The pay-off came first. And the prodigality of the supporting structure is all the more fantastic for having been, necessarily, erected downwards.

The temptation, when reading the tales, is to follow this course, to jump to the last line and find out where on earth we're travelling. This would be... (what dolorous misadventure would this be?)... a sad mistake. The punchline, such as it is, needs to be earned: arrive too soon, and you'll wonder why you bothered. Better, far better, to travel marvellously.

And there you have them, The Various Lives of Keats and Chapman. Not art for art's sake, perhaps: but extravagance for the sheer joy of extravagance. Caveat lector.

· Edited extract from Jamie O'Neill's introduction to The Various Lives of Keats and Chapman and The Brother by Flann O'Brien, published by Scribner/TownHouse